Evacuations of the coast are already being recommended along vulnerable areas of Massachusetts’ Cape Cod Bay, as a developing low-pressure system centered over Indiana on Thursday morning heads towards New England. The storm was bringing a swath of accumulating snow to southern lower Michigan, where up to 9” is expected. By Thursday night, when the low will be in western Pennsylvania, the system will undergo a complex interaction with a short-wave trough of low pressure to its north, resulting in the formation of a secondary area of low pressure--a nor’easter--off of the Northeast U.S. coast. This nor’easter will intensify dramatically to about a 975 mb central pressure on Friday, bringing a wide range of hazards to New England. Heavy rains of 2 – 3” in Rhode Island and Southeast Massachusetts will bring river flooding; heavy wet snows in excess of one foot in the higher elevations of New England will cause power outages; wind gusts in excess of 60 mph will affect the coasts of Connecticut, Rhode Island and Massachusetts, with hurricane-force wind gusts of 75 mph expected to occur on Cape Cod; and a storm surge of up to 4’ (with 25 - 35-foot waves) will cause major coastal flooding and erosion along the coast of eastern Massachusetts. A lesser storm surge of 2 – 3’ will affect the coasts of New Hampshire and Maine, causing mostly moderate flooding.

Figure 2. Predicted maximum significant wave height for the 3-day period ending on March 3, 2018, from the SWAN wave model (driven by winds from the NAM model). Waves in excess of 30 feet are expected offshore from Cape Cod, Massachusetts, and in excess of 25 feet offshore from Boston. Image credit: CERA.

Two 1-in-100-year coastal flooding events in one year for Boston

According to NOAA, there is a 1% chance per year that Boston will see coastal flooding in excess of 4.59’ above Mean Higher High Water, MHHW (a 1-in-100-year event). A flood of this magnitude already occurred once this year, on January 4, when Winter Storm Grayson brought a top-five highest storm tide on record to the coast from Boston to Central Maine. Boston saw its highest water level on record, 4.88’ above MHHW. The NWS in Boston is predicting a storm surge of 2.5 – 3.5 feet during the time of high tide on Friday and Saturday. Exacerbating the event will be the highest tides of the month—more than a foot above average--associated with the full moon. The combined effect of the storm surge and the unusually high astronomical tide is predicted to bring a water level among the top-three highest ever measured, about 4.5’ above MHHW. Thus, this weekend’s storm surge may well give Boston a second 1-in-100-year coastal flooding event, something one would expect to see randomly only once every 10,000 years!

Figure 3. Predicted water levels in Boston due to the combined effects of the tide and storm surge (black line with “X” marks), along with the expected water level due just to the tide (blue line), using a storm surge model driven by the 6Z Thursday, March 1, 2018 winds from the GFS model. The difference between the black curve and the blue curve is the storm surge (gold line). MLLW, MSL, MHHW, and MAT stand for Mean Lower Low Water, Mean Sea Level, Mean Higher High Water, and Maximum Astronomical Tide. MAT is the maximum tide that will occur in a 19-year span, and there is probably flooding if the total water level crosses MAT. Boston is predicted to see a storm tide about 4 – 4.5’ above MHHW, which would be the third highest water level ever recorded there. Fortunately, it appears the highest storm surge of 4’ will arrive at low tide, or else Boston would see an all-time record water level. The record high water level was set earlier this year--4.88’ above MHHW on January 4, 2018 (previous record 4.82’ on 2/7/1978 during the infamous Blizzard of ’78). Boston’s period of record (POR) extends back to 1921. Image credit: NWS.

A long-duration and large-scale nor’easter

Unfortunately, a strong and slow-moving ridge of high pressure (a “blocking high”) over Greenland, which has brought an incredible flow of warm air northwards into the Arctic with above-freezing temperatures at the North Pole, will keep this weekend’s nor’easter from making much progress eastward. As a result, the Northeast U.S. coast will receive a punishing assault from a large storm surge and high waves that will last through three high tide cycles.

In Eastern Massachusetts, the worst flooding is expected to occur during the high tide cycles near 11 am Friday, 11 pm Friday and 12 pm Saturday, when waves of 25 – 35 feet are possible just offshore. This stretch of coast is unusually vulnerable to erosion, thanks to the terrific pounding wrought by the storm surge and wave action of Winter Storm Grayson on January 4. In one case, Winter Storm Grayson helped wash away 60 – 75% of 110,000 cubic yards of sand added in 2016 to Town Neck Beach (about 40 miles southeast of Boston). This leaves the beach highly vulnerable to a renewed assault by this weekend’s storm.

From weather.com’s write-up of the impacts of this nor’easter: On Long Island, New York, coastal flooding could result in numerous road closures and could flood homes, business, and parking lots near the coast.

In Manasquan, New Jersey, Saturday morning's storm tide could reach levels that begin to flood the first floor of low-lying homes and businesses, levels only topped the past 25 years by Hurricane Irene (2011), the December 1992 Nor'easter, and Superstorm Sandy (2012).

Moderate to major coastal flooding is possible in the Virginia Tidewater, including the Norfolk metro area, Saturday morning, evening, and even Sunday morning.

In the Outer Banks of North Carolina, oceanside and soundside flooding Saturday into Sunday may inundate stretches of Highway 12 near times of high tide.

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